ENH: dedicated HashSetOps, HashTableOps namespaces

- relocated HashSetPlusEqOp and HashTablePlusEqOp to
  HashSetOps::plusEqOp and HashTableOps::plusEqOp, respectively

- additional functions for converting between a labelHashSet
  and a PackedBoolList or List<bool>:

  From lists selections to labelHashSet indices:

      HashSetOps::used(const PackedBoolList&);
      HashSetOps::used(const UList<bool>&);

  From labelHashSet to list forms:

      PackedBoolList bitset(const labelHashSet&);
      List<bool> bools(const labelHashSet&);